1. Field of the Invention
The present invention generally relates to a method of driving a liquid crystal display apparatus, and, more specifically, to a method of driving such a liquid crystal display apparatus having a matrix pixel structure of a ferroelectric liquid crystal.
2. Description of Prior Art
In general, a liquid crystal is classified into three types of liquid crystals according to their molecule arrangement. Among these liquid crystals, there is a smectic liquid crystal. When a ferroelectric liquid crystal representing a C-phase among the smectic liquid crystals is injected into a space formed between two glass substrates, twisted molecule axes are returned, transparent electrodes being formed inside the glass substrates. As a result, optic axes of the ferroelectric liquid crystal are aligned in a predetermined direction, and the optic axes of the liquid crystal molecules are varied by influences of the electric field produced between these transparent electrodes to which a driving voltage is being applied. As is well known in the art, when a driving voltage is applied to the transparent electrodes in a panel constructed by sandwiching the above-described two glass substrates between two polarizing plates, this panel can hold a display function in which the optical birefringence thereof is changed and then the optical modulation occurs.
Such a display apparatus employing the ferroelectric liquid crystal can represent a higher response than a TN (twisted nematic) type liquid crystal display apparatus employing a nematic liquid crystal. It is also known in the art that a dot-matrix display having a large display area with higher resolution can be achieved in a smectic type liquid crystal display apparatus, which, to the contrary, cannot be achieved in a TN type liquid crystal display apparatus. The latter display apparatus utilizes the display storage characteristics.
Since a display apparatus employing a ferroelectric liquid crystal has, however, a clear relationship between its electrooptic effect and its applied voltage, as compared with a display apparatus using a twisted nematic (TN) liquid crystal, a specific driving method is required to operate a ferroelectric liquid crystal type display apparatus. The electrooptic effect is understood in that when an electric field is applied to the liquid crystal, astable phenomena such as arrangement transition of a molecule rate and a molecule flow of the liquid crystal, occurs, and the optical characteristic of the liquid crystal changes. Moreover, the ferroelectric liquid crystal type display apparatus has another characteristic of dependence on the pulse width of the applied voltage and of temperature dependence.
According to the specific driving method, both the selected condition and non-selected condition of the display are changed by applying a pulsatory driving voltage having positive (+) and negative (-) polarities, as illustrated in FIG. 1, and by changing a sequence of the polarity of the driving voltage applied to the liquid crystal.
Since in the above driving method, the liquid crystal molecules slightly responds to either the waveform of the biasing voltage, or the voltage pulse of intermediately selected conditions, the selected condition of the display cannot be maintained if the driving voltage increases, and the display contrast is lowered. Accordingly, various problems may occur in that the operation margin (i.e., a ratio of the minimum selected voltage to the maximum nonselected voltage) becomes lower, and in addition, a relatively higher driving voltage of approximately .+-.20V is required.
The present invention is therefore made in consideration with the conventional drawbacks, and has an object to provide a method of driving a liquid crystal display apparatus in which even if the liquid crystal cell can be driven by a relatively lower voltage, a satisfactory display contrast can be achieved.